1. Field of the Invention
The present invention relates to a metal compound solution and a process for producing a thin film using the same. More particularly, it relates to a metal compound solution comprising a specific organic solvent which is useful as a raw material of chemical vapor deposition and a process for producing a lead titanate-based thin film using the same.
2. Description of the Related Art
A thin film of a composite metal oxide having controlled composition and structure is expected for application to semiconductors, electronic components, optical materials, and the like. Inter alia, lead titanate-based composite oxides, such as lead titanate (PT), lead zirconate titanate (PZT), and lead lanthanum zirconate titanate (PLZT), exhibit specific electrical characteristics, such as a large specific dielectric constant and ferroelectricity, and have been used in electronic devices, e.g., highly dielectric and/or ferroelectric memory devices.
Processes for making a thin film of the composite metal oxide include sputtering, ion plating, pyrolytic coating, and so forth. Among them chemical vapor deposition (hereinafter xe2x80x9cCVDxe2x80x9d) has been given studies as the most suitable thin film formation technique because of ease of composition control, excellent stop coverage, and adaptability to the semiconductor production processes.
Materials fit for CVD are compounds which can supply a metallic element constituting a desired metal oxide in a vapor phase, i.e., a metal compound that vaporizes stably. Metal compounds which are usually used include alkyl compounds, alkoxide compounds and complexes with xcex2-diketone compounds. Where the metal compound is a solid having a high melting point, a solution CVD technique, in which a solution of the metal compound in an organic solvent is used, has been proposed for securing sufficient supply of the metal source.
In making a lead titanate-based thin film by CVD, solution CVD is adopted because xcex2-diketonatolead complexes that are solid are mostly used.
The organic solvent which can be used in solution CVD includes polar solvents that are highly capable of dissolving metal compounds to produce a stabilizing effect, such as tetrahydrofuran, butyl acetate, and polyethylene glycol dimethyl ether. For example, Japanese Patent Laid-Open Nos. 7-76778 and 10-298762 disclose formation of a lead titanate-based thin film by solution CVD using a tetrahydrofuran solution. Unlike the other metal compounds, however, the xcex2-diketonatolead complexes are liable to form white turbidity or a precipitate in a polar solvent such as tetrahydrofuran, causing disorders in the thin film formation. To address this problem, Jpn. Appl. Phys., vol. 38, pt. 1, No. 9B, pp 5326-5330 (1999) reports production of a lead titanate thin film by solution CVD using a hydrocarbon solvent such as octane. Although these organic solvents do not form white turbidity or a precipitate, they are less capable of dissolving the metal compound, easily causing the dissolved solid material to precipitate out with temperature change or partial volatilization of the solvent or concentration change of the solution, failing to guarantee stability for use.
An object of the present invention is to provide a metal compound solution having stability and a concentration fit for solution CVD, a CVD material comprising the solution, and a process of producing a thin film using the CVD material.
As a result of extensive investigation, the present inventors have found that the above object is accomplished by a certain kind of cyclohexane compounds which are capable of sufficiently dissolving metal compounds to provide a stable CVD material.
The present invention has been reached based on the above finding. The invention relates to a metal compound solution comprising a metal compound represented by formula (I): 
wherein R1, R2, and R3 each represent a halogen-substituted or unsubstituted alkyl group having 1 to 8 carbon atoms which may contain an oxygen atom in its carbon chain; R4 represents a straight-chain or branched alkylene group having 2 to 18 carbon atoms; M represents a lead atom, a titanium atom or a zirconium atom; when M is a lead atom, l, m and n represent 0, 2 and 0, respectively; when M is a titanium atom or a zirconium atom, n represents 0 or 1, and l and m each represent an integer of 0 to 4 satisfying equation: 1+m+2n=4,
the metal compound being dissolved in a cyclohexane compound represented by formula (II): 
wherein R5 represents a hydrogen atom or an alkyl group having 1 to 4 carbon atoms; and p represents 1 or 2.
The present invention provides a metal compound solution having stability and a concentration fit for solution CVD, a CVD material comprising the solution, and a process of producing a thin film using the CVD material.